1. Field of the Invention
The present invention relates to a method of manufacturing a pneumatic tire taken measures concerning electric resistance problem by forming an electrically conductive layer on a tread surface and forming an electrically conductive sidewall rubber. The present invention also relates to the pneumatic tire.
2. Description of the Related Art
Conventionally, there are known pneumatic tires that include a tread rubber blended with silica at a high ratio to reduce rolling resistance, which has a close relationship with fuel consumption efficiency of vehicles, as well as to increase braking performance on wet road surface (wet braking performance). However, such tread rubber has the following disadvantage. That is, compared to a tread rubber blended with carbon black at a high ratio, the tread rubber blended with silica has a higher electrical resistance. Therefore, electrostatic generated on vehicle body and/or tires is prevented from being released therefrom, thereby causing a problem such as radio noise. In order to solve this problem, the following pneumatic tires have been developed. That is, an electrically non-conductive tread rubber blended with silica and the like is provided with an electrically conductive layer blended with carbon black at a high ratio to eliminate the electric resistance problem.
For example, U.S. Pat. No. 5,518,055, Japanese Unexamined Patent Publication (Kokai) No. 10-81110, Japanese Unexamined Patent Publication (Kokai) No. 10-203114, and Japanese Unexamined Patent Publication (Kokai) No. 2007-8269 describe the following pneumatic tires. That is, an outer peripheral surface of an electrically non-conductive tread rubber located in an edge area thereof is covered with an electrically conductive layer, and the electrically conductive layer is disposed on a boundary between a tread rubber and a wing rubber or a sidewall rubber. An edge portion of the electrically conductive layer is arranged to be in contact with a rim or an electrically conductive rubber portion, which is electrically conductive from the rim (for example, wing rubber or sidewall rubber); thereby the electrostatic generated on a vehicle body and/or tires is released to the road surface.
However, in these tires, the electrically conductive layer is widely exposed to the outer peripheral surface in an edge area of tread rubber. Therefore, these tires have the following problem. That is, when the tread rubber wears to a certain extent, the electrical conductivity performance hardly functions and the electrical conductivity performance is not maintained satisfactorily. Moreover, since the electrically conductive layer is formed in a sheet-like configuration, the volume of the electrically conductive layer becomes excessively large. Moreover, since the electrically conductive layer is continuously exposed on the tread surface, improvement effect obtained by using the electrically non-conductive tread rubber is not exerted satisfactorily. That is, improvement effect of fuel efficiency performance and wet braking performance, obtained by the tread rubber blended with silica at high ratio, cannot be achieved satisfactorily.
U.S. Pat. No. 2,339,546 describes a pneumatic tire in which an electrically conductive layer extends along a carcass layer, one end of which is exposed to a tread surface through an inside of a tread rubber and the other end thereof is in contact with a rim. The electrically conductive layer has a sheet-like configuration continuously extends in a circumferential direction of the tire. As a result, the volume of the electrically conductive layer is excessively large as described above, and the electrically conductive layer is continuously exposed to the tread surface. In such a case, improvement effect of the electrically non-conductive tread rubber used may not be obtained satisfactorily. Japanese Unexamined Patent Publication (Kokai) No. 2007-176437 describes a pneumatic tire, an electrically conductive layer of which extends spirally toward the tread surface from a rim strip rubber (clinch rubber). In this case, an electrically conductive path is excessively long.
Further, Japanese Unexamined Patent Publication (Kokai) No. 11-217011 and Japanese Unexamined Patent Publication (Kokai) No. 11-78417 describe the following pneumatic tires. That is, the pneumatic tires includes an electrically non-conductive tread rubber and a shoulder rubber joined with a sidewall rubber or an extended sidewall rubber, which covers a surface of a shoulder portion, an end portion of which extends up to a ground contact end region of the tread rubber. The sidewall rubber and the shoulder rubber are formed of an electrically conductive rubber, and the end portion of the sidewall rubber is in contact with a rim.
However, generally, sidewall rubbers are poor in wear resistance. Therefore, it is not desirable to allow the sidewall rubber to come in contact with road surface in initial stage of wear. Further, in these tires, the sidewall rubber blended with carbon black at a high ratio is widely exposed to an outer peripheral surface in an end area of the tread rubber. An Effect, that is obtained by using a tread rubber blended with silica at a high ratio; i.e., an effect to improve fuel saving performance and wet braking performance may not be obtained satisfactorily.